This invention relates to a movable flight control surface on an aircraft, and more particularly to an apparatus for sealing the gap between the control surface and the airfoil on which it is mounted, when the control surface is in a deflected position, thereby decreasing the aerodynamic drag of the aircraft.
Referring now to FIGS. 1 and 2 of the drawing, FIG. 1 illustrates an aircraft 10 of a type known in the art, such as an MD-11.RTM. jetliner, manufactured by McDonnell Douglas Corporation of Long Beach, Calif. The aircraft has a pair of airfoils or wings 12, on which are mounted a plurality of engines 14. The wings each further include a leading edge 16 and a trailing edge 18.
In order to maintain flight control of the aircraft, it is known in the art to employ movable control surfaces along the trailing edge 18 of each aircraft wing 12. For example, flaps 20 on each wing 12 may be extended rearwardly to one or more deflected positions spaced from the trailing edge 18, in order to increase lift during take-off and drag during landing, thereby permitting the aircraft to fly at lower than normal speeds without stalling. Similarly, outboard ailerons 22 and inboard ailerons 24 may be extended rearwardly to one or more deflected positions spaced from the wing trailing edge 18 in order to control the aircraft's rolling and banking movements during flight.
In state of the art aircraft, when a movable control surface is in a deflected position on the airfoil, streamwise gaps are formed between each edge of the control surface and corresponding edges of the airfoil. With particular reference to prior art FIG. 2, this is illustrated for the case of an outboard drooped aileron 22, though it is equally applicable to other movable control surfaces, such as flaps or inboard ailerons, as well. Thus, as illustrated, a gap 26 is formed between the outboard edge 28 of the aileron 22 and a corresponding first edge 30 of the airfoil. A similar gap is formed on the inboard side of the aileron as well, between the aileron inboard edge 32 and a corresponding second airfoil edge 34. These resultant gaps permit air to flow therethrough, as illustrated by the arrows 36, 38. The air leakage reduces the effectiveness of the movable surface, and correspondingly increases the aerodynamic drag of the aircraft.
With modern large jet transport aircraft, any measurable drag penalty, even if only a fraction of one percent, will result in significant additional fuel costs, amounting to many thousands or even millions of dollars fleetwide during the course of an operating year. In the highly competitive airline industry, where profit margins are often razor thin, this can make the different between solvency and insolvency. Even more importantly, such drag penalties, especially if they are cumulative with other drag penalties, such as underperforming engines, may cause a range shortfall for the aircraft which is significant enough to prevent it from effectively flying non-stop routes for which it may have been designed or acquired. Such a range shortfall, and the resultant loss of ability to fly certain long range non-stop routes may place an aircraft at a severe competitive disadvantage.
Current solutions for reducing the gap between movable flight control surfaces and the airfoil on which they are mounted involve the use of flexible seals on the edges of the surface or the fixed airfoil. The seals are designed to reduce the gap between the airfoil and the movable surface when the surface is not deflected, but they do nothing to prevent the above discussed airflow leakage when the movable surface is deflected.
Thus, what is needed is a solution for the problem of airflow leakage through the gaps between a movable control surface and its associated airfoil when the control surface is in an extended deflected position, thereby reducing aerodynamic drag and consequently improving the flight effidency of the aircraft.